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External Loads
Words of caution about what you strap to the floats
Written by Arvid Weflen
All went well as the pilot of the floatplane lifted off the
water with a load of one-quarter-inch plywood tied to the spreader bars. When he leveled
off for cruise, the aircraft felt as if the brakes were being applied. In order to avoid
stalling. the pilot pushed the stick full forward.
| Lumber, canoes, moose -- the variety of external
loads carried in Alaska is almost as great as the variety of floatplanes in use, but a permit
is required. |
With the nose of the aircraft quite vertical, the airplane was
going only one direction -- straight down at a high rate of descent.
By a fantastic stroke of luck, there was a small lake directly
under the airplane and the landing was hard but safe.
During the few minutes it took for the pilot to regain his
composure, the cause of the exhilarating flight became apparent. The front of the thin
sheets had not been secured to each other, allowing them to fan out and create a tremendous
amount of drag. Another lesson learned.
Pilots being what they are, external loads will be strapped,
tied or otherwise fastened to the outside of airplanes in an attempt to transport them to
another location. If it won't fit inside the plane, people will fasten it to the outside.
Many pilots routinely and legally carry snowshoes, skis and
rifles on the lift struts of some aircraft. Others, not quite so legally, have carried
snowmachines strapped to the belly of their aircraft or to the lift struts beside the
fuselage.
But the best type of plane to carry oversize loads is a
floatplane. With its multitude of perfectly located struts, tying odd-shaped pieces of
lumber, boats and furniture onto it seems natural.
Every summer, I find myself fastening a variety of items
onto the floats of my Piper Super Cub for one reason or another. Last summer, it was mostly
lumber and other building materials for a new cabin.
The loads have come to seem rather commonplace to me, but
spectators at the airport will hang around for hours to watch to see if the floatplane
will fly with all that stuff tied on it.
| Caribou antlers may be a light load, but
on floats they disturb the airflow over the elevators. Bedsprings and Christmas trees also
create interesting turbulence. |
Carrying external loads produces varied results -- a number
of trips will be very ordinary and then there'll be the trip that causes your pulse to
rise to new heights.
Yet, if carefully planned, most flights can be conducted in
relative safety. Contrary to common lore, obtaining a permit from the Federal Aviation
Administration for external loads -- which is required before strapping things on -- is
relatively easy, at least in Alaska. A maintenance inspector will review your
qualifications, discuss procedures and issue a restricted certificate and a list of
limitations.
While the limitations were derived from the experience of
previous pilots, they may not apply to every type of aircraft in every situation, as one
pilot of a Cessna 185 learned. He had carried a canoe occasionally without incident until
the day he decided to carry a canoe on each float. He made it, but swore he would never do
that again. Carrying two canoes can be permitted, but in this case, for whatever reasons,
the aircraft had not flown even close to normally.
Learning to fly with external loads is usually a self-taught,
trial-and-error process. Since flight characteristics vary with differing loads, the pilot
must be thoroughly familiar and current in the aircraft used. The FAA requires the pilot to
meet the hour requirements for a commercial certificate and have at least 50 hours of
pilot-in-command time in the make and model of airplane. I feel these times are extreme
minimums.
When first carrying loads that change the certificated shape
of your aircraft, start small and build up to larger loads slowly. Small loads, such as a
couple boards, usually will have little effect on most aircraft.
Other than noticing the water splash up onto the boards and
spray out a little differently, about the only change will be the worried look on the pilot's
face -- wondering when the knots will become untied, allowing the lumber to fly into the
tail, or if there will be any changes during landing. After completion of the first flight,
he will wonder why he was so worried.
As my loads grew larger, I used to worry about individual
boards in the center of a stack slipping out. Thus, before a night, I would carefully nail
them all together before tying the heavy, unwieldy stack of boards to the struts.
Later, I discovered there is surprisingly little force on
individual boards or other items on the floats. I Learned that the day I absentmindedly
left a small board lying on one float and found it still there after landing one hour later.
Since then, I haven't bothered to nail the boards together. Never once has even one board
shifted.
To fasten a stack of boards to the struts, I use three pieces
of one-quarter-inch nylon rope, each about eight feet long. Two pieces are used to tie the
stack to the front strut and one for the rear. I use two pieces on the front since I am
basically chicken. If one comes untied, I will still have the other to hold the load firmly
to the aircraft. So far, none have ever come loose.
Large loads lower the cruise speed surprisingly little. My
Super Cub on floats usually cruises around 97 mph and is slowed to 93 to 95 mph with a
large load. Since floatplanes travel so slowly anyway, no one really complains much about
a few miles per hour. No one, but no one, flies a floatplane in order to travel fast.
A large load of lumber on each float handles nicely in flight,
but if the ball on your turn coordinator gets very far off center, it tends to stay off to
that side or even shift a little farther. Your instructor's admonishments to keep the ball
centered return to haunt you at these times. In other words, it really does matter that the
ball is centered.
It seems logical that since most aircraft require holding
right rudder on takeoff, external loads should be loaded primarily on the right float.
However, I haven't been able to notice much difference regardless which side has the larger
load. A gross imbalance of the load will require holding the appropriate rudder, but about
the same amount of rudder is needed in either case. I do try to carry an equal-sized load
on each float.
Everyone I know who carries lumber prefers anything except
plywood. The plane wallows through the air and just feels funny. I suppose there really
isn't that much difference in how the plane feels, but when you have many hundreds of hours
in the same plane, you are fairly well attuned to the aircraft and slight differences are
very noticeable.
Another problem with plywood is that the takeoff run is
always long. The 4- by 8-foot sheets will fit only between the floats on the spreader bars
on most aircraft. Plywood does fit there nicely, but the rear edge of the plywood drags
through the water while the floatplane is getting on the step. On a hot calm day, the effect
of this extra drag can be frustrating and unsuccessful takeoff attempts. While some wind
will always shorten the takeoff run even a 3-mph breeze will make a big difference under
these circumstances. Once on the step, acceleration and takeoff performance is about the
same as it would be without an external load.
Cruise speed with plywood is close to normal, but because of
the slight change in flight characteristics, I have a strong preference for flying on days
with little or no turbulence.
Tying plywood onto spreader bars requires a fair amount of
rope and lots of time. It helps if you don't mind getting a little damp in the process of
crawling around in the 2- or 3 foot-high space between the belly of the aircraft and the
water's surface. Unfortunately, the lakes in Alaska are all rather chilly and getting damp
usually means getting chilly also.
I use at least three pieces of one-quarter-inch-diameter rope.
One rope is used across the top of the plywood on each of the spreader bars to hold the
plywood down firmly. The third rope is run from one end of the rear spreader bar diagonally
around the rear edge of the plywood to the other end of the spreader bar to keep the plywood
from shifting rearward.
The last part of fastening plywood onto the floats involves
using a portable drill to bore a few small holes in the front edge of the plywood to fasten
all the sheets together with safety wire. I normally do this with all thicknesses of plywood
even though it probably isn't really necessary for sheets one-half inch or thicker. The
last thing I want to happen is having the sheets fan out in the breeze during flight.
Most flights with external loads are relatively uneventful.
But once in a, while, peculiar results occur. An aluminum ladder tied to a float seems
like an innocuous load. But it disrupted the airflow over the elevators and made the aircraft
very sensitive in pitch requiring fairly constant trim corrections.
Moose antlers are bulky and awkward and rarely fit inside
small aircraft. The antlers can be carried on the wing struts, but this places heavier loads
on the struts than I like and creates a large amount of asymmetrical drag. However, tying
antlers to the floats doesn't present too many problems if you don't fall into the lake trying
to fasten the ropes.
You will definitely feel the asymmetrical drag, but as long
as the flight doesn't drag on for hours, you should not get a cramp in your leg from
continually pushing on the rudder pedal. The odd shape of the antlers does cause some
disturbance of the airflow over the elevators, however, creating a slight vibration.
I have known of really large loads such as dressers and
bookshelves being carried on floats. In these instances, the plane looked oddly undersize
in comparison with the load.
One pilot who carried such a load remarked that getting
off the water was easy since it was so light, but once in the air the airplane just wouldn't
accelerate to normal speeds. Usually getting off the water is the biggest problem and
flight is fairly normal.
One large potential problem with external loads is the
possibility of loading the aircraft out of center of gravity limits. Enlist the help of
your mechanic, who is familiar with aircraft weighing procedures, to help determine station
numbers for the individual rivet rows on the floats. Then use these known locations for
load placement and calculate CG as usual.
Eyeballing the load will get you close, but I know of at
least one incident when that wasn't close enough.
The pilot fastened a rear quarter of a moose on each float,
guessing that the CG was about right. Takeoff went okay but when he pulled the wheel back
to climb, it reached the stops, proving that the estimated CG location wasn't a very good
guess.
After wondering, "How did I ever get into this?" the next
thought of the pilot was, "Should I crash out here where the FAA might not find out about
it, or in town where there will be more help available?"
By this time, he was over the trees and figured since he
was still in the air, he may as well crash in town. When reaching his destination, a
higher-than-normal approach speed enabled him to maintain control and the landing was
uneventful -- other than the additional gray hairs that developed on his head en route to
town.
"Why bother carrying external loads, considering the known
risks?" I've been asked. Rest well assured, if I have another choice, I will do it that way,
but sometimes there is no practical alternative, especially in the more remote parts of the
world where roads and rivers don't always go where you want to go.
Really large aircraft on floats, such as a DC-3, are not
exactly common, but Cessna's new turbine-powered Caravan on floats sounds big enough to
eliminate the need for external loads. On second thought, I bet I could haul a really big
boat on its floats.
Source: 1985 Water Flying Annual
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